U.S. Pat. No. 4,062,463 discloses a unique transport system which employs a single cassette from which workpieces, such as silicon wafers, are transferred to a processing station and, after processing, are returned to the same cassette. Such a system eliminates the need for two separate cassettes, and, hence, it also eliminates the necessity of providing a rather complex mechanism for coordinating the movement of the cassettes such that one is positioned to transfer the workpieces to the processing station while the other is positioned to receive the workpieces after they have been processed. Because it employs a single cassette and a relatively simple transfer mechanism, the system of the '463 patent can be made rather compact. Despite its advantages, the system disclosed in the '463 patent does suffer from one drawback. Namely, such a system can only process one workpiece at a time.
The systems which employ two cassettes, one for delivering workpieces to a processing station and the other for receiving the processed workpieces from the processing station, are advantageous in that they can be more readily adapted to process a plurality of workpieces simultaneously. This advantage has, as indicated above, been offset by the need for rather complex mechanisms to coordinate the movement of the two cassettes. Typically, such a mechanism would require two separate elevators, one for each cassette.